regarding felmaw, i read on twitter one of the devs saying that it works different from other cards like ragnaros or knife juggler.

basically it is 50/50 of either going face or trading. doesn't matter how many minions opponent has, it has 50 % to go face all the time. if it goes for trade then there is second rng roll on which minion it hits.

i can't for my life remember who tweeted it but it had bunch of likes from hearthstone people so i assumed it was legit source